The invention relates to the carbonylation of ethylene using carbon monoxide in the presence of a catalyst system and to such a catalyst system.
The carbonylation of ethylene using carbon monoxide in the presence of an alcohol or water and a catalyst system comprising a Group III metal, e.g. palladium and a phosphine ligand, e.g. an alkyl phosphine, cycloalkyl phosphine, aryl phosphine, pyridyl phosphine or bidentate phosphine, has been described in numerous European patents and patent applications, e.g. EP-A0055875, EP-A-04489472, EP-A-0106379, EP-A-0235864, EP-A-0274795, EP-A-0499329, EP-A-0386833, EP-A-0441447, EP-A-0489472, EP-A-0282142, EP-A-0227160, EP-A-0495547 and EP-A-0495548. In particular, EP-A-0227160, EP-A-0495547 and EP-A-0495548 disclose that bidentate phosphine ligands provide catalyst systems which enable higher reaction rates to be achieved.
The main problem with the previously disclosed catalyst systems is that, although relatively high reaction rates can be achieved, the palladium catalyst dies off quickly which necessitates the frequent replenishment of the catalyst and hence results in a process which is industrially unattractive.
It has now been found that a particular group of bidentate phosphine compounds can provide remarkably stable catalysts which require little or no replenishment; that use of such bidentate catalysts leads to reaction rates which are significantly higher than those previously disclosed; that little or no impurities are produced at high conversions.
Accordingly, the present invention provides a process for the carbonylation of ethylene which process comprises reacting ethylene with carbon monoxide in the presence of a source of hydroxyl groups and of a catalyst system, wherein the catalyst system is obtainable by combining:
(a) a metal of Group VIII or a compound thereof; and
(b) a bidentate phosphine of general formula (I) 
xe2x80x83wherein
R0 is a tertiary carbon atom
each of R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11 and R12 is independently a pendant optionally substituted organic group which carries a carbon atom through which the group is linked to the respective R0;
each of L1 and L2 is independently a linking group selected from an optionally substituted lower alkylene chain connecting the respective phosphorus atom to the group X; and
X is a bridging group comprising an optionally substituted aryl moiety to which the phosphorus atoms are linked on available adjacent carbon atoms.
In a second aspect, the present provides a catalyst system capable of catalysing the carbonylation of ethylene, which catalyst system is formed from
(a) a metal of Group VIII or a compound thereof; and
(b) a bidentate phosphine of general formula (I) 
xe2x80x83wherein
R0 is a tertiary carbon atom
each of R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11 and R12 is independently a pendant optionally substituted organic group which carries a carbon atom through which the group is linked to the respective R0;
each of L1 and L2 is independently a linking group selected from an optionally substituted lower alkylene chain connecting the respective phosphorus atom to the group X; and
X is a bridging group comprising an optionally substituted aryl moiety to which the phosphorus atoms are linked on available adjacent carbon atoms.
The pendant optionally substituted organic groups, R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11 and R12, may be independently selected from a wide range of components. Preferably, the pendant groups are optionally substituted lower alkyl, e.g. C1-8, and which may be branched or linear.
Particularly preferred is when the organic groups, R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11 and R12, when associated with their respective R0 carbon atom form composite groups which are at least as sterically hindering as t-butyl. Steric hindrance in this context is as discussed at page 14 et seq of xe2x80x9cHomogeneous Transition Metal Catalysisxe2x80x94A Gentle Artxe2x80x9d, by C Masters, published by Chapman and Hall, 1981.
The linking groups, L1 and L2, are independently selected from an optionally substituted, particularly lower alkyl, e.g. C1 to C4, substituted, lower alkylene, e.g. C1 to C4 chain. Especially preferred is when both L1 and L2 are methylene.
The bridging group X is an aryl moiety, e.g. a phenyl group, which may be optionally substituted, provided that the two phosphorus atoms are linked to adjacent carbon atoms, e.g. at the 1 and 2 positions on the phenyt group. Optional substitution of the aryl moiety may be by other organic groups, e.g. alkyl, particularly C1-8, aryl, alkoxy, carbalkoxy, halo, nitro, trihalomethyl and cyano. Furthermore, the aryl moiety may be a fused polycyclic group, e.g. naphthalene, biphenylene or indene.
Examples of suitable bidentate ligands are bis(di-t-butyl phosphino)-o-xylene (also known as 1,2 bis(di-t-butylphosphinomethyl)benzene), bis(di-t-neopentyl phosphino)-o-xylene and bis 1,2(di-t-butyl phosphino) naphthalene. Additionally, the bidentate phosphine may be bonded to a suitable polymeric substrate via at least one of the bridging group X, the linking group L1 or the linking group L2, e.g. bis(di-t-butyl phosphino)-o-xylene may be bonded via the xylene group to polystyrene to give an immobile heterogeneous catalyst.
The amount of bidentate ligand used can vary within wide limits. Preferably, the bidentate ligand is present in an amount such that the ratio of the number of moles of the bidentate ligand present to the number of moles of the Group VIII metal present is from 1 to 50, e.g. 1 to 10 and particularly from 1 to 5 mol per mol.
The carbon monoxide may be used in the presence of other gases which are inert in the reaction. Examples of such gases include hydrogen, nitrogen, carbon dioxide and the noble gases such as argon.
The process of the present invention is preferably carried out at a temperature from 20 to 250xc2x0 C., in particular from 40 to 150xc2x0 C. and especially from 70 to 120xc2x0 C.
The process may be conducted under a total pressure of from 1xc3x97105 to 100xc3x97105 N.mxe2x88x922 and in particular from 5xc3x97105 to 50xc3x97105 N.mxe2x88x922.
Suitable Group VIII metals include cobalt, nickel, palladium, rhodium and platinum. Particularly preferred is palladium. Suitable compounds of such Group VIII metals include salts of such metals with, or compounds comprising weakly coordinated anions derived from, nitric acid; sulphunc acid; lower alkanoic (up to C12) acids such as acetic acid and propionic acid; sulphonic acids such as methane sulphonic acid, chlorosulphonic acid, fluorosulphonic acid, trifluoro methane sulphonic acid, benzene sulphonic acid, naphthalene sulphonic acid, toluene sulphonic acid, e.g. p-toluene sulphonic acid, t-butyl sulphonic acid, and 2-hydroxypropane sulphonic acid; sulphonated ion exchange resins; perhalic acid such as perchloric acid; perfluororated carboxylic acid such as trichloroacetic acid and trifluoroacetic acid; orthophosphoric acid; phosphonic acid such as benzene phosphonic acid; and acids derived from interactions between Lewis acids and Broensted acids. Other sources which may provide suitable anions include the tetraphenyl borate derivatives. Additionally, zero valent palladium compounds with labile ligands, e.g. tri(dibenzylideneacetone) dipalladium, may be used.
The catalyst system of the present invention may be used homogeneously or heretogeneously. Preferably the catalyst system is used homogeneously.
The catalyst system of the present invention is preferably constituted in the liquid phase which may be formed by one or more of the reactants or by the use of a suitable solvent.
Suitable solvents that may be used in conjunction with the catalyst system include one or more aprotic solvents such as ethers, e.g. diethyl ether, dimethyl ether, dimethyl ether of diethylene glycol, anisole and diphenyl ether; aromatic compounds, including halo variants of such compounds, e.g. benzene, toluene, ethyl benzene, o-xylene, m-xylene, p-xylene, chlorobenzene, o-dichlorobenzene, m-dichlorobenzene, and p-dichlorobenzene; alkanes, including halo variants of such compounds, e.g. hexane, heptane. 2,2,3-trimethylpentane, methylene chloride and carbon tetrachloride; nitrites, e.g. benzonitrile and acetonitrile; esters, e.g. methyl benzoate, methyl acetate and dimethyl phthalate; sulphones, e.g. diethyl sulphone and tetrahydrothiophene 1,1-dioxide; amides, including halo variants of such compounds, e.g. dimethyl formamide and N-methyl pyrrolidone.
The catalyst system of the present invention is particularly suited to the carbonylation of ethylene. Surprisingly, propane has been found to be difficult to carbonylate to the extent that the present catalyst system may be viewed as not being able to carbonylate propane.
The end product of the reaction is determined at least in part by the source of hydroxyl groups that is used. The use of water gives rise to the corresponding carboxylic acid whereas the use of an alkanol leads to the corresponding ester. Suitable alkanols include C1-30 alkanols, optionally substituted with one or more substituents such as halogen atoms, cyano, carbonyl, alkoxy or aryl groups. Suitable alkanols include methanol, ethanol, propanol, 2-propanol, 2-butanol, t-butyl alcohol and chlorocapryl alcohol. Particularly useful are methanol and ethanol.
The molar ratio of the amount of ethylene used in the reaction to the amount of hydroxyl providing compound is not critical and may vary between wide limits, e.g. from 0.001:1 to 100:1 mol/mol.
The product of the reaction may be separated from the other components by any suitable means. However, it is an advantage of the present catalyst system that significantly fewer by-products are formed thereby reducing the need for further purification after the initial separation of the product as may be evidenced by the generally significantly higher selectivity. A further advantage is that the other components which contain the catalyst system which may be recycled and/or roused in further reactions with minimal supplementation of fresh catalyst.